Zinc base casting alloy

ABSTRACT

IMPROVED ZINC BASE DIE CASTING ALLOYS CONFORMING TO A.S.T.M. B86-64 AG40A WITH LOWERED MAGNESIUM CONTENTS THAT IMPART IMPROVED FLUIDITY PROPERTIES ENABLING LOWERED TEMPERATURE DIC CASTING USE AND RETAINING DIMENSIONAL STABILITY AND PERMANENCE OF PROPERTIES AFTER AGING.

3,679,404 Patented July 25, 1972 United States Patent Oflice 3,679,404 ZINC BASE CASTING ALL Y Leslie J. Larrieu, San Marino, Califl, asslgnor to MOI'I'IS P. Kirk 8: Son, Inc., Los Angeles, Cahf. No Drawing. Filed Feb. 8, 1971, Ser. No. 113,728 Int. Cl. C22c 17/00 US. Cl. 75-178 AM 8 Claims ABSTRACT OF THE DISCLOSURE Improved zinc base die casting alloys conforming to A.S.T.M. B86-64 AG40A with lowered magnesium contents that impart improved fluidity properties enabhng lowered temperature die casting use and retaining dimensional stability and permanence of properties after aging.

quirements:

Aluminum 3.5-4.3 Copper (max.) .25 Magnesium .02-.05 Iron (max.) .10. Lead (max.) .005 Cadmium (max.) .004 Tin (max.) .003 Zinc Remainder This A.S.T.M. Alloy AG40A of B86-64 enjoys a very large usage in the manufacture of precision parts by the pressure die casting process. This alloy possesses satisfactory fluidity, castability, mechanical properties, properties of dimensional permanence and good resistance to intercrystalline oxidation, but: it does suffer from reduced castability because of the presence of .02% to .05 magnesium in its composition.

This Alloy AG40A evolved from the discovery that the presence of copper in the alloy composition in amounts of 1.0% detracted from dimensional stability and retention of ductility as evidenced by impact strength and dimensional measurements after long term periods of aging.

The amounts of magnesium heretofore prescribed a range of .01% to 30% and a preferred amount of .10%. This preferred amount of .10% was cited as a necessary safe amount to prevent intercrystalline oxidation often referred to as intergranular corrosion.

Alloy AG40A is manufactured by most alloyers today with magnesium in the range .035 to .045% in order to meet all specifications, some of which require a minimum of .03 in the cast product.

I have discovered that Alloy AG40A must have a minimum amount of magnesium in its composition such as .002%-003% otherwise the resulting alloy is soft, weak and dimensionally unpredictable. I have discovered that Alloy AG40A containing magnesium in the range of about .002% to about 009% are very satisfactory alloys possessing very good dimensional stability properties after long term periods of aging and are free of intercrystalline oxidation after exposure to moist atmospheres at 200 F. for 100 hours.

I have further discovered that the presence of copper in my alloy in an amount of a trace to about .50%, with an intermediate range of about .0l% to about .25 and a preferred amount of about .10% contributes to the castability as well as the increased tensile strength and Brinell, hardness of the resulting alloy both in the as cast condition and after long term aging. Also, that the mechanical properties of the AG40A Alloy containing magnesium in this range are similar in retained amount to those of the AG40A Alloy containing .02% to .04% magnesium after long term periods of aging.

Further, the AG40A Alloy containing this range of magnesium is die castable at a temperature of 25 F. to 35 F. lower than the equivalent AG40A containing .03% to .04% magnesium. An intermediate range of magnesium of about .003% to about .007% and a preferred amount of about .005% have been used and it has been found that they possess the same properties as those set forth above.

It has been further discovered that it is not necessary to add other metallic elements such as nickel to this new alloy in order to ensure stability and permanence of properties. Magnesium alone in the ranges heretofore set forth accomplishes this result.

Typical representations of the revealed discoveries are presented in tabular form with the following brief description of the mechanical and physical data presented in each table.

Table 1.-The effect of various levels of magnesium upon the mechanical properties of AG40A both as cast and aged 7 days at 150 F.

Table 1A.--The effect of various amounts of magnielsium on the fluidity and hence castability of the alloy. a oy.

Table 2.-The effect of magnesium in amounts up to .15% upon the mechanical properties of the AG40A Alloy both as cast and aged 30 days at 200 F.

Table 3.The effect of copper upon the mechanical properties of the AG40A Alloy containing .009% magnesium both as cast and aged 7 days at 150 F.

Table 4.-The effect of copper upon the mechanical properties of the AG40A Alloy containing .002% magnesium and the same alloy containing .02% magnesium both as cast and aged 60 days at 150 F.

Table 5.The eifect of copper upon the physical and mechanical properties of the AG40A Alloy containing .002% magnesium and the same alloy containing .02% magnesium including dimensional changes, percent gain in weight for the individual test specimens, impact strength and Brinell hardness properties of each composition after exposure to a moist atmosphere of 200 F. for hours.

Table 6.-The eflect of .003 magnesium and the effect of .041% magnesium upon the mechanical and dimensional properties of the AG40A Alloy containing moderate amounts of the soft metals, lead, tin and cadmium after various terms of aging.

Table 7.The comparative mechanical and dimensional properties of four AG40A type alloys after aging periods of 2 /2 years. Alloys include compositions without magnesium, with .005% magnesium, with .03% magnesium and with both .005% magnesium and .50% copper.

LEGEND FOR DATA TABLES Tensile strengths are quoted in lbs. per sq. inch. Elongation is listed as percent increase in length of 2" gage length. b Impacts are quoted in ft. lbs. determined on a A" x A" 1 ar- Bhn is an abbreviation for Brinell hardness number. Compositions include all element quotations in percent. R.T. is an abbreviation for room temperature.

' 3 Aged properties with temperature are all dry heat conditions.

As cast properties are determined 24-48 hours after casting.

4 the compositions. The lot 7576 containing no magnesium flowed into the second ring a linear distance of 4%". Taking this figure as 100%, the other compositions tested flowed the linear length set forth or percent of the linear TABLE 1 Composition As cast properties Aged 7 days at 150 F.

Elon- Elon- Al Cu Mg Pb Sn Cd Tensile gation Impact Bhn Tensile gation Impact Bhn TABLE 1A distance travelled by the magnesium free alloy. The full The usual casting temperature for the alloy AG40A is from 800" F. to 825 F. The liquids temperature of the alloy is 733 F. and the solidus temperature is 717 F.

In making the fluidity test, a mold having a series of concentric rings is used. The outer ring is connected to the sprue and has a gate to the next inner succeeding ring at a point opposite'to the sprue with gates connecting each inner succeeding ring at points opposite to the gate con- .nection with the next outer ring.

composition of the alloys tested are set forth in Table 1 under the same lot numbers.

Length of N0. 2 ring, in. Percent Lot:

The usual casting temperature for the Alloy AG40A castability of the low magnesium content alloys relative to the higher magnesium content set forth in the specificawere300 to 310 F. for the ring mold and 800 F. for tion for the AG40A Alloy.

TABLE 2 Composition As cast properties Aged 30 days at 200 F.

Elonga- Elonga- Lot Al Cu Mg Pb Sn Cd Tensile tion .Impaet Bhn Tensile tion Impact Bhn TABLE 3 Composition As cast properties Aged 7 days at 150 F.

Mg Pb Sn Cd Tensile Elong. Impact Bhn Tensile Elong. Impact Bhn TABLE 4 Composition As cast properties Aged days at 200 F.

Elonga- Elonga- Mg Pb Sn Cd Tensile tion Impact Bhn Tensile tion Impact Bhn TABLE 5 Moist atmosphere at 200 F. for Composition hours Dimensional change I Gain 1 Percent Impact Bhn I for 2" wt. after alter 3 years 60 da Lot Al Cu Mg Pb Sn Cd length gain steam s at R.'I. at

1 Test specimen 54" x 34" x 2". 8 Test specimen wt.14.8 grams.

TABLE 6 Dimensional Composition As cast properties Aged 216 years at T.R. Aged 6 months at 200 F. change Elon- Elon- Elon- 2 2 h Tenga- Im- Tenga- Im- Tenga- Ime53 yearsi Lot Al Cu Mg Pb Sn Cd sile tion pact Bhn sile tion pact Bhn sile tion pact Bhn at LT. 200F 1257.- .4.-10 0 003 0065 0057 0047 39, 799 5. 0 37. 5 79. 6 36, 126 4. 0 39. 0 69. 1 32,452 7. O 35. 0 60. 5' 0065 .002 1211;- 4. 05 0 041 0081 0049 0057 39, 799 4. 0 38. 5 79. 6 37,759 5. 0 39. 5 74. 1 32, 961 6. 0 34. 2 60. 5 006 001 *Specimens7.9 inches in length.

TABLE 7 Composition Aged 2% years at RT. Dimensional change 2% years 2% years Lot Al Cu Mg Pb Cd Sn Tensile Elong. Impact Bhn at RJI. at 200 F.

Change in length for bar 7.9" long.

The results presented in Table 1 confirm the elfects of a very small amount of magnesium upon the AG40A Alloy both in the as cast and the aged conditions. The as cast data confirm the concept that the mechanical properties of the alloys with .0045% magnesium to .009% magnesium are adequately satisfactory and further, that this adequacy is amply confirmed by the data presented for the properties obtained by accelerated aging for 7 days at 150 F.

The mechanical property results presented in Table 2 emphasize the role of magnesium in the AG40A Alloy since it is quite evident that not much is to be gained by using .02% magnesium rather than 006% magnesium in the composition of the alloy. The larger additions such as .10% magnesium and .15% magnesium yield mechanical property results both as cast and aged that are detrimental.

With regard to the consideration of strength properties, it should be remembered that zinc base alloys of the AG40A type are strongest in the as cast condition and that a true evaluation of these alloys should, in large measure, be based upon the strength properties retained after moderate to long term periods of aging. For instance, the .006% magnesium content alloy in Table 2 is very equivalent in all mechanical properties to the .02% magnesium content alloy after an accelerated aging period of 30 days in a dry atmosphere of 200 F.

The role of small additions of copper to the AG40A Alloy is one of hardening and strengthening. For instance, in Table 3 the alloy with .15 copper with magnesium at .009% is stronger in terms of tensile strength than the similar alloys with .11% copper and .07% copper.

This strengthening role of copper when present in small amounts such as .10% is additionally confirmed by the mechanical test data listed in Table 4. Although the increases are not large they are significant.

Table 5, as in Table 4, lists a series of AG40A Alloys with the bare minimum of .002% magnesium and the large addition of .02% magnesium as well as no copper and .10% copper. An examination of Table 5 reveals the fact that regardless of the amounts of magnesium and copper present, all compositions are satisfactory after exposure to a moist atmosphere of 200 IF. after 100 hours. Intercrystalline oxidation was not detected for any of the specimens including the composition containing the bare minimum amount of .002% of magnesium and a total absence of copper.

This sameseries of alloys was also subjected to moderate room temperature aging periods and the dimensional properties of all compositions were found to be eminently satisfactory as can be seen by examination of the data in Table 5. The dimensional properties for all compositions after 60 days aging at 150 F. were also quite equivalent and satisfactory. Thus, it will be seen from the results obtained in the tests set forth in Table 5 that the difference between the use of the bare minimum of .002% magnesium and the larger amount of .02% magnesium are equivalent.

Intercrystalline oxidation, sometimes called intergranular corrosion is a very destructive process and has been feared by all who are familiar with zinc alloy metallurgy. Some metallurgists believe that this destructive process is related to phase changes and that elements such as copper and magnesium separately and together retard phase changes and also act beneficially in resisting intercrystalline oxidation. Later researchers in this field of destructive corrosion have almost conclusively placed the blame upon the presence of the soft metals such as lead, tin and cadmium.

During the introductory years of the zinc =base die casting alloys it was difficult to produce and obtain zinc metal with high purity and the introductory alloys consequently contained substantial amounts of lead and cadmium in their compositions. The very high purity of zinc metal available in more recent years has eliminated the spectre of lead contamination and along with it intercrystalline oxidation. Average electrolytic Special High Grade Slab Zinc available today does not contain an amount greater than .005 of combined lead, cadmium and tin, and it is expectable by the consumers of zinc base die casting alloys that the alloys supplied to them will not exceed this combined total percent.

Table 6 presents the comparative mechanical and dimensional properties of two AG40A Alloys, one of which contains .003% magnesium and the other .041% magnesium and with both alloys containing moderate amounts of the soft metal contaminants, lead, cadmium and tin. The as cast mechanical properties, the aged properties after 2% years at room temperature and the longer term properties after aging 2% years at 200 F. indicate an almost exact equivalency for the two alloys. The test data also reveal very similar dimensional changes after moderate term aging at room temperature and longer term aging at 200 F. Thus, demonstrating the eflect of the presence of low magnesium content on possible intercrystalline oxidation from moderate amounts of the soft metal contaminants.

Table 7 presents the mechanical properties for four AG40A Alloys after aging 2 /2 years at room temperature and the dimensional properties after aging 2 /2 years at room temperature and also after aging at the more severe conditions of 2 /2 years at 200 F. The composition include no magnesium, .005 magnesium, 03% magnesium and .005 magnesium with .50% copper.

The aged mechanical properties after 2 /2 years at room temperature confirm the weakness of the AG40A Alloy without magnesium. The equivalency of of mechanical properties for the 005% magnesium and the .03% magnesium is evident. The copper addition in an amount of .50% to the .005% magnesium alloy expectably improves the retention of tensile strength and hardness.

The dimensional properties for the two magnesium bearing alloys after aging 2 years at room temperature and after aging for the longer term of 2 /2 years at 200 F. are essentially equivalent. The copper hearing alloy shows some expectable dimensional changes due to the etfect of copper. The magnesium free alloy reveals an objectionable amount of expansion after long term aging at 200 F., thus confirming the necessity to have some magnesium present in the AG40A Alloy.

Thus it has been amply demonstrated that Alloy AG40A containing a magnesium range of .002% to .009% and a preferred amount of .005 both with and without copper in the ranges set forth is an eminently satisfactory alloy for die casting purposes with regard to all mechanical and dimensional properties and will permit the die casting industry to enjoy all of the advantages of lower temperature use, which are very de sirable from the standpoint of the die casting industry.

Although the Alloy AG40A recites a range of aluminum as 3.5 to 4.3%, these zinc base alloys have been with a wider range of aluminum such as 3% to 5%.

All percentages referred to herein are by weight.

What is claimed is:

1. A zinc base casting alloy of high purity containing low amounts of magnesium having superior fluidity and hence improved castability with good mechanical prop- "weight essentially of about 3.5% to about 4.3% aluminum, about .003% to about .007% magnesium and the remainder zinc with iron not in excess of .10% and the combined impurities of lead, cadmium and tin not in excess of .012% total.

3. A zinc base casting alloy of high purity containing low amounts of magnesium having superior fluidity and hence improved castability with good mechanical properties and dimensional stability after aging, consisting by 7 weight essentially of about 4% aluminum, about .005 v magnesium and the remainder zinc with iron not in excess of .10% and the combined impurities of lead, cadmium and tin not in excess of .012% total.

4. A zinc base casting alloy of high purity containing low amounts of magnesium having superior fluidity and hence improved castability with good mechanical properties and dimensional stability after aging, consisting by weight essentially of about 3% to about 5% aluminum, about .002% to about .009% magnesium, trace to about .50% copper and the remainder zinc with iron not in excess of .10% and the combined impurities of lead, cadmiumand tin not in excess of .012% total.

5. A zinc base casting alloy of high purity containing low amounts of magnesium having superior fluidity and hence improved castability with good mechanical properties and dimensional stability after aging, consisting by "weight essentially of about 3.5% to about 4.3% aluminum, about .003% to about .007% magnesium, about .01% to, about .25 copper and the remainder zinc with iron not in excess of .10% and the combined impurities of lead, cadmium and tin not in excess of .012 total.

6. A zinc base casting alloy of high purity containing low amounts of magnesium having superior fluidity and hence improved castability with good mechanical properties and dimensional stability after aging, consisting by weight essentially of about 4% aluminum, about 005% magnesium, about .10% copper. and the remainder zinc with iron not in excess of .10% and the combined impurities of lead, cadmium and tin not in excess'of .012% total.

7. A zinc base casting alloy of high purity containing low amounts of magnesium having superior fluidity and hence improved castability with good mechanical properties and dimensional stability after aging, eomisting by weight essentially of:

[Percent Aluminum 3-5 Magnesium .002.009 Iron, max. .05 Lead, max. .003 Cadmium, max. .002 Tin, max. .001 Zinc Remainder 8. A zinc base casting alloy of high purity containing low amounts of magnesium having superior fluidity and hence improved castability with good mechanical properties and dimensional stability after aging, consisting by weight essentially of:

L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner us. 01. X.R. 7s 17s A 

